I've read this several times. I understand it bit better every time so I'll see I I can break it down a bit.
Unmyelinated type II afferent neurons report cochlear damage
An experiment was performed where cells were ruptured and from I can get from the research, the supporting cells were deemed to be a major source of ATP acting on type II afferents.
Such immediate rupture of individual hair cells might occur in vivo. More commonly, however, acoustic stress progressively damages OHCs, leading to their eventual death, and is known to increase ATP concentration in cochlear fluids in vivo.
This work provides direct evidence that type II afferents, in addition to sensing glutamate release from OHCs, are activated by cochlear damage in the young rat's cochlea. This observation may help to resolve the decades-long conundrum that type II afferents in vivo are very insensitive to sound and yet presumably carry some information to the auditory brainstem. Likewise, measured ex vivo, synaptic excitation is weak and could activate type II afferents only if all of the presynaptic OHCs were maximally stimulated. Alternatively, ATP potently activates type II afferents and serves as a major contributor to the damage-induced response. ATP can be released into cochlear fluid after tissue stress (even without OHC ablation) in vitro, or noise exposure in vivo.
I'm a bit confused by the above, I've tried to make some sense of it. It seems to suggest that in vivo or vitro, there is relatively low chance of Type II sensitization, but however if all of the associated hair cells become maximally stimulated then it will become sensitized. But also it alternatively seems to suggest that upon noise exposure, importantly not necessarily leading to hair cell death, ATP could still be released and cause Type II sensitization. I could well be reading this wrong though but it's what it seems to say.
Experimental ablation of OHCs was shown to initiate ATP-dependent calcium waves in nearby Hensen's (support) cells that further triggers release of ATP through their connexin hemichannels. P2X2 receptors have been located to the postsynaptic junction in the OHC region in adult guinea pig, and P2Y2 receptors have been identified in a small population of spiral ganglion neurons in both adult and neonatal rats, suggesting the expression of purinergic receptors in type II neurons.
I understand that
expression of purinergic receptors in type II neurons means the receptors respond to ATP.
Of interest in this context is the previous report that sensitivity to ATP is reduced in type II afferents after the onset of hearing, consistent with the fact that loud sound is not usually painful to normal ears. However, purinergic signaling in the cochlea is up-regulated after noise exposure, raising the possibility that type II afferents become more sensitive after damage, in part by increased sensitivity to ATP.
This suggests to me that as we know, loud sound for the majority of people isn't painful and that the more the Type II afferents are exposed to ATP, which is more and more frequently after repeat noise exposure, the more they are stimulated by ATP, and then they become more and more sensitive to ATP until they finally become sensitized, at which point they do begin to transmit pain signal upon noise exposure. It seems to not be suggesting that its a one time switch where ATP automatically sensitizes a Type II afferent but cumulative damage.
What I seem to be getting from reading this again, is that it might be suggesting that Type II afferent can become sensitized without OHC death necessarily occurring. If I'm missing something obvious please let me know.
Type I afferents are strongly activated by glutamate release from IHCs, but not by ATP. Type II afferents are strongly activated by ATP, but only weakly by glutamate release from OHCs. These distinctions reinforce the hypothesis that type I and type II afferents serve different functional roles: as acoustic (type I) versus trauma (type II) detectors.
Finally, the KCNQ activator retigabine can silence both type II afferents and somatic pain fibers.
That experiment you posted last week about the deaf mice still haunts me though because I wish they'd tested what repeat noise exposure would do after the hair cell death, because this question seems to be becoming more prominent now, its obvious that upon hair cell death ATP is released, and according to the above maybe even not necessarily upon hair cell death. What I'd love to know is what kind of activity occurs after the hair cell death. I don't believe Ive ever seen a study showing ATP being released over and over from support cells upon repeat noise exposure, ideally also measuring the specific frequencies used and the support cell response to each individual frequency.